The invention relates to a scanning particle microscope comprising a particle beam generator and comprising at least one additional lens in the particle beam path.
Scanning electron microscopes are being employed to an increasing degree in the semiconductor industry in the manufacture of integrated circuit components for the inspection of micro-electronic components or of articles that are required in the manufacture of micro-electronic components, for example for the inspection of masks and wafers. The employment of a scanning electron microscope in the measurement of electric potentials is known, for example, from U.S. Pat. No. 4,277,679. The employment of a scanning electron microscope for generating microstructures by means of electron beam lithography is known, for example, from U.S. Pat. No. 4,075,488. Over and above this, a scanning electron microscope can also be employed for monitoring the individual process steps in the manufacture of an integrated electronic component, for example in order to check length dimensions, or the positioning of a mask or wafer.
In mensurational applications of a scanning electron microscope such as, for example, in the measurement of spatial dimensions or in the measurement of electric potentials, care must be taken that the test results are not falsified due to charging of the surface of the object under test. In order to avoid a charging of a surface of an object under test, the surface of the object can be rendered conductive by vapor deposition with conductive material. In measurement applications wherein a vapor deposition of the surface with conductive material is not possible due to required further processing of the object to be investigated, as is usually the case, for example, for objects under test in micro-electronics, it is generally necessary that the primary electrons impinge on the surface of the object with such energies that, on chronological average, the charge leaving the surface of the object (e.g. as secondary electron emission) is exactly equal to the charge applied to the surface. The energies of the incident primary electrons at which a charge balance is achieved between charge impinging on the surface and charge departing from the surface are generallly relatively low and typically lie in the range from about 500 eV to two keV. For the case of special operating modes wherein the incident charge has to be compensated for only as an average result over the course of time, or where the object under test is composed of special materials, an adequate charge balance or avoidance of charging of the surface of an object under test can still be achieved even for impinging energies of the primary electrons up to ten keV and above.
In electron beam printers, the energies of the primary particles impinging on the surface of an object are presently on the order of about twenty keV. However, the tendency can be observed that lower energies of the primary particles are likely to be desired in the future in electron beam printers. Such lower energies are desirable since the lower the energy of the primary particles incident on an object, the lower is the proximity effect, and the lower is the scatter volume in the resist and in the target. Consequently in the future there will be a need not only for printers having finer and finer particle beam probes with higher and higher particle beam current, but also for progressively lower energies of the impinging particles. For a conventional scanning electron microscope, these desirable objectives are contradictory, since the lower the energy of the particles in the particle beam, the lower is the resolution of measurements utilizing such conventional scanning electron microscopes, the larger is the diameter of the particle probe incident on the target, and the lower is the particle beam current. Conventional scanning electron microscopes are therefore not optimally adapted for anticipated high production high resolution printing and measurement applications. In conventional scanning electron microscopes the cause of the difficulties is the so-called Boersch effect (H. Rose, R. Spehr, Advances in Electronics and Electron Physics, Supplement 13c, 1983, pages 475-530) which opposes focussing of the particles in the particle beam. The brightness of, specifically, high-intensity particle sources (such as, for example, lanthanum hexaboride single crystal cathodes for generating electron beams) can therefore not be fully exploited. Particularly given low energies of the particles, the brightness decreases due to the Boersch effect along the beam path from the particle source to the target on which the particles impinge. Given an electron beam having an energy of the electrons of one keV, the loss in brightness along the path from the electron source to the target can exceed a factor of twenty. For the case where the particles are of low energy, the crossover points of the particle beam that lie beyond the various lenses along the beam path are considerably broadened (increased in area) under certain conditions, this leading to a broadened (increased area) of the particle probe, to a deterioration of a mensurational resolution and to a reduction of the particle current density.
German OS 31 38 926 discloses an electron microscope for generating high-current probes comprising a single-stage probe shaping system utilizing a short focal length imaging lens. Since H. Rose et al have shown in "Optik" 57, 1980, No. 3, pages 339-364, that the energy spread of the primary electron beam rises proportionally with the number of electron beam crossover points, the Boersch effect is diminished in the electron microscope of this German OS No. 31 38 926. Such a known electron microscope, however, is not suitable for applications in which a large working interval is required between the lens system and the target, or in which a blanking system is required that is disposed at a beam crossover point that is relatively easily accessible and that must have a relatively large interval from the target so that detrimental effects of the potentials present in the blanking system on the target are avoided.
German OS No. 32 04 897 discloses a particle beam generating system that is designed as a tetrode. According to this disclosure, a sub-system of the particle beam generating system, composed of cathode, Wehnelt electrode and anode, is operated such that for a specific particle beam energy, optimum field intensities and, thus, an optimum brightness are achieved. In order to obtain different particle beam energies, the difference in potential between this sub-system and the additional, following electrode can be set to different values. In this way, a particle beam having an optimum brightness and a desired particle beam energy is generated in the particle beam generating system. With this known particle beam generator, however, the optimum brightness existing immediately following the particle beam generator cannot be prevented from being reduced over the further particle beam path due to the Boersch effect.